Turbofan engines operate with an high level of safety, reliability and efficiency, which is a credit to the engine designer who is challenged to design an engine to operate under numerous conditions and to address any contingency. One such contingency event is known as a fan blade out event.
The fan is a vital component of a turbofan engine such as those that are commonly found on aircraft. The fan will have fan blades that extend radially outwardly from a central hub or disk. Fan designs differ primarily in the manner in which the blades are structurally tied or connected to the disk. In one fan design type, a dovetail connection is used to connect the fan blade and an associated shank to the disk. In another fan design type, the fan blade airfoil itself, without a shank, is welded to the disk. This later design is sometimes referred to as a bladed disk (or BLISK) fan, as the blades and disk form an integrated unit.
The fans operate at high rotational speed, and the fan blades themselves experience significant operating stresses, particularly in a radial direction extending away from the disk. In the fan blade out event, the fan blade or a portion of the fan blade separates from the disk and is discharged into the engine case. The design challenge is to ensure that fan blade fragments and other detritus are contained by the engine case during a fan blade out event.
In a typical fan blade out event, the fan blade separates at a minimum cross-sectional thickness region adjacent the disk. The result is the release of the fan blade airfoil part and the shank part, if a dovetail design, from the disk. The released fan blade interacts with at least the first trailing blade and the surrounding engine case.
In the case of a dovetail design, because of the presence of the platform and shank, the released blade interacts with the first trailing blade and the containment case simultaneously. Because of the blade profiles, the first trailing blade tends to draw the released blade aft-wards, i.e., into the engine axially. Whereas the impact force on the released blade due to its interaction with the containment case tries to push the released blade forward due to the conical shape of the containment case. In a dovetail because of the significant interaction of the released blade with first trailing blade, the released blade tends to move in a direction into the engine.
In aircraft engines with a BLISK type fan blade design, the fan blades are frictionally welded to the disk at the root. Hence there is no shank present at the root of the fan blade as would be found in a dovetail type fan blade design. In a fan blade out event in an engine with a BLISK fan blade design, it is more likely an airfoil part of the blade will separate from the disk than for the disk itself to separate or disintegrate. It has been observed that as much as 80% or more of the full blade airfoil may separate in a fan blade out event of a BLISK fan.
The separated airfoil portion of a BLISK fan blade, even as much as the entire fan blade, has less mass and therefore less inertia than a dovetail design fan blade, which includes the shank and attaching platform. Moreover, the absence of a shank and attaching platform from the BLISK fan blade moves the center-of-gravity (CG) of the released airfoil portion radially outwardly from the disk. A result of these differences is that in a fan blade out event in a BLISK fan engine, the released blade part to first trailing blade interaction occurs much later after blade release. By the time of interaction with the first trailing blade, which tends to draw the released blade into the engine, the desired direction, as much as or more than 50% of the released blade part will have engaged the engine case. Engagement of the blade part with the engine case tends to move the blade part toward the engine inlet and potentially out of the engine case.
Therefore, it is desirable to provide an engine case design for turbofan engines that has the effect of retaining fan blade parts within the engine case in a fan blade out event. This may be accomplished by an engine case design that tends to direct fan blade parts into the engine after a blade out event. Other desirable features and characteristics of the herein described embodiments will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.